I don't know exactly if I got you right @muhammadblibla.
If you have a free trial with the purchase of Stadia, you can simply redeem it and after the free trial you will be automatically charged for Stadia Pro, if you don't cancel. The cancellation you can do from the first day on after your trial started. Here are some more information in general about the Stadia free trial.
If this was not what you were asking: Do you mind explaining a bit more in detail?